By Isaac Stanley-Becker,

HAMBURG — The Group of 20 economic summit began Friday in this northern German port city after clashes between protesters and police that could send ripples through the two days of meetings.

German security forces used water cannons and pepper spray on Thursday to clear an anti-capitalist march in which a group with anarchist sympathies had a prominent presence. The skirmish followed an hour-long standoff adjacent to Hamburg’s harbor, where protesters were attempting to move from a public square toward the downtown conference center where Germany’s chancellor, ­Angela Merkel, is hosting foreign leaders, including President Trump. A concert in the evening will take place at the Elbphilharmonie, a crown jewel of a city that is among the country’s most affluent and yet is burdened by higher-than-average unemployment.

Left-wing activists keen on drawing attention to climate change, labor rights and the plight of refugees — among other causes — vowed to interrupt the first day of summit proceedings. They planned to blockade three principal sites around the city, using civil disobedience to frustrate an attempt by authorities to cordon off swaths of Hamburg linking the summit locations to hotels and the airport.

At stake are questions about security, free expression and democratic assembly — newly relevant alongside a summit that, while traditionally devoted to economics, may also showcase different approaches to human rights and the rule of law. Merkel, who is chairing the summit, said she will highlight climate, free trade and the shared obligation to succor refugees.

Her critics say her policies are part of the problem.

“This week is about Angela Merkel’s austerity policy going global via G-20,” said Jan van Aken, a member of the German Parliament representing the far-left Die Linke party.

He criticized the German government for seeking to control protest, saying its approach was autocratic and would “make Erdogan, Putin and Trump feel at home here” — a reference to Turkish leader Tayyip Erdogan and Russian President Vladi­mir Putin, as well as the American leader.

The government is sensitive to this point.

“The main issue is that the summit is again, after Brisbane, in a democracy,” said Wolfgang Schmidt, a Hamburg politician involved in summit preparations. Summits in Turkey and China followed the 2014 meeting in Australia. “You want to make sure that protest and dissenting views are heard, but you also need to maintain security, and with 42 highly protected heads of state and finance and foreign ministers, it’s a challenge.”

The security challenge came into sharp focus Thursday night. When police attempted to separate a group of “black bloc” activists from the roughly 12,000 people who had assembled to protest inequality and economic greed, authorities met a hail of rocks and bottles. As police rushed the group, some of the protesters fled. But a phalanx of activists dressed in dark clothes, with their faces concealed, held their ground. They carried signs that condemned the state and declared “Welcome to hell.”

Police made some arrests but said Thursday evening they did not have a final tally. They said 15 officers were injured, two of whom were hospitalized. A furniture store and a bank were damaged, police said.

Medics could be seen treating the injured on the sidelines of the demonstration.

The street marches planned for the summit — emulating the forceful dissent seen at past G-20 gatherings — cover a range of issues, including calls for environmental protection, denunciations of ethnic nationalism and opposition to free trade.

But the Hamburg protests have gained added momentum as a stand against Trump and his brand of “America First” populism. An estimated 100,000 protesters were expected to converge on the old merchant city during the summit.

Meanwhile, 20,000 officers were being deployed at about 30 registered demonstrations in the largest police operation in Hamburg’s history. Forty-five water cannons were available to disperse crowds, and a no-fly zone was in place over portions of the city.

Peter Grant, a 68-year-old self-identified Communist who moved from the United States to Germany 45 years ago to study, said he was worried that Trump’s presidency was encouraging far-right groups in Europe, a continent riven in the 20th century by nationalism. He said 20 world leaders, cloistered in a conference center, “have no right to decide on the future of humanity.”

A 33-year-old woman studying education, Katalina, who declined to give her last name, said the problem was bigger than Trump.